The present invention relates generally to the art of photoconductive devices and, more particularly, to a photoconductive device containing a zinc oxide transparent conductive layer.
Photoconductive devices generally consist of a photoconductor capable of generating an electrical potential upon being exposed to light and contacts which are effective to draw off any electric current which results from irradiation of the photoconductor. In most instances, such photoconductive devices also contain a suitable substrate such as glass, in order to provide protection from the environment and to serve as a base for the photoconductor. Many different photoconductive materials are known, such as silicon, germanium, gallium arsenide and copper indium diselenide, to name but a few. Photoconductors comprising silicon have obtained particularly wide usage because of their economy. Originally, single crystal silicon photoconductors were widely used. However, recently thin film alloys of silicon and hydrogen (TFS) have come to be preferred because of their lower cost and ease of fabrication. In particular, TFS layers have been utilized in photoconductors which incorporate a P-layer, an I-layer and an N-layer, which are known as P-I-N photoconductors. The I-layer, the intrinsic layer, is generally formed from a thin layer of microcrystalline or amorphous silicon alloyed with hydrogen or a silicon hydride. The P-layer is formed from silicon doped with boron or other dopant which will enable positive charge carriers, known as holes, to be produced at the interface between the P-layer and the I-layer. The N-layer is formed from silicon doped with phosphorous or other suitable material which will enable electrons to be separated at the interface between the N-layer and the I-layer. Contacts on the P-layer and the N-layer permit electric current to be drawn off into an external circuit for use therein. More recently, such contacts have taken the form of thin films, such as films of tin oxide or indium tin oxide, at least in the case of the front contact, that is the contact adjacent to the transparent substrate which may face or be distant from the incoming light rays. The back contact generally has been fabricated from various metals such as aluminum, silver, molybdenum, titanium or nickel. Such metals, however, are transparent to light rays only when they are applied in extremely thin layers. Tin oxide and indium tin oxide, on the other hand, are relatively transparent in reasonable thicknesses. However, even with such relatively conductive materials as the above-mentioned oxides, it has been found that a certain loss of electrical energy results in these layers. Moreover, such materials are fairly expensive.
It is accordingly an object of the present invention to provide a photoconductive device in which a relatively inexpensive transparent conductive layer or layers is utilized.
Another object of this invention is to provide a photoconductive device having desirable characteristics.
Other objects and advantages of the present invention will become apparent from the following detailed description.